Cell mutants defective in DNA replication and/or other functions involved in cell growth have been isolated. A class is temperature-sensitive (ts) for semi-conservative cell and polyoma DNA replication. One, TS-2, is being studied in detail in an effort to determine the biochemical basis for the ts defect. A human fibroblast has been transformed by a fragment of SV40 DNA corresponding to the early region, including the origin of DNA replication. These transformants are producing large quantities of free, circular viral DNA.